The invention relates to a method for protecting an intermediate circuit capacitor in a power converter circuit having at least two submodules connected in a series circuit and receiving via an inductance electrical power from a DC voltage side of a power source, a control device for executing the method, a power converter circuit having the control device, and an electric motor having the power converter circuit.
A power converter circuit for supplying electrical power to the motor windings of an electric motor in a vehicle drive system is known for example from the publication Lukas Lamberts et al, “Modularer Hochfrequenz Umrichter für Fahrzeugantriebe” (modular high-frequency converter for vehicle drive systems), EMA 2010, 8 to 9 Sep. 2010, Aschaffenburg.
The circuit in question is one which is intended for converting a DC voltage from an electrical power source into a plurality of AC voltages. The individual AC voltages are generated by submodules connected in series, at the input side of each of which a part of the DC voltage drops. Within the submodules the corresponding part of the DC voltage is in each case converted by way of a single-phase full bridge operating as an inverter into an AC voltage which can be delivered to one of the motor windings. The single-phase full bridge comprises two single-phase half bridges, each of which is provided for generating an AC voltage phase, which means that the two AC voltage phases in total yield a single-phase AC voltage.
The partial voltages dropping at the inputs of the individual submodules are increased prior to the inversion by the full bridge by means of a boost converter. The boost converter comprises an inductance, which is connected in series between the electrical power source and the series circuit of the full bridges, and also an input half bridge in each submodule. The full bridge and the half bridge are connected on the DC voltage side within the submodule.
Connected in parallel with the full bridge and with the input half bridge in each submodule is an intermediate circuit capacitor which can store electrical energy, for example from a reactive power flow from the motor windings.
EP 2 369 725 A1 discloses a bridging unit having a bistable relay for converter cells connected in series. By means of the relay, in the case of an excess current and/or an excess voltage, but also if an external closure signal is present, the inputs of the respective converter cell are short-circuited and a low-impedance conduction path for bridging the converter cell is thereby made available. In this case an electric current present at the inputs flows by way of the bridging unit and not through other structural components of the converter cell. A semiconductor switch can be switched in parallel with the relay in order to shorten the response time of the bridging unit.